Method and a Device for Removing Gas from Wood Chips

ABSTRACT

A method and a device for removing gas from wood chips, wherein the wood chips flowing downwards are heated with a steam flow that is cross-directional in relation to the wood chip flow.

FIELD OF THE INVENTION

The invention relates to a method for removing gas from wood chipsaccording to the preamble of the appended claim 1. The invention alsorelates to a device for implementing the aforementioned method inaccordance with the preamble of the appended claim 12.

BACKGROUND OF THE INVENTION

When manufacturing cellulose and paper pulp, theligno-cellulose-containing wood chips used as raw material are cooked inan alkaline solution in order to separate the fibres and lignincontained in them from each other. The manufacture comprises severalstages, both before and after the cooking. Before cooking, the woodchips are directed to the gas removal phase, where gases, primarily air,are removed from both inside the wood chips and the spaces between themby directing hot steam to the wood chips. Removing gas from the spacesbetween the wood chips decreases the amount of gases disturbing theoperation of the digester. In addition, when the gases are removed frominside the wood chips, the chips absorb cooking chemicals better, whichimproves the yield of the cooking and the quality of the pulp itprovides.

Vertical bins or vessels or horizontal screws are generally used forremoving gas from wood chips. The screws being used are generally woodchip conveyor screws, which convey wood chips, for example, fromchipping to the digester. Generally, this type of screws are installedhorizontally or only in a small angle with the horizontal level. Thesteam needed for removing gas is generally fed from nozzles installed onthe lower surface of the shell forming the bottom of the screw. However,the loading of the screws varies in relation to time, i.e. in relationto the length of the screw, the amount of wood chips moving insidevaries, resulting in slow warming of individual wood chips to themaximum temperature and varying their delay in the screw. Consequently,the processing of the wood chips is not uniform. In addition, the delayof the wood chips in the screw is not long enough in order for the gasremoval to be complete.

When using vertical bins or vessels in removing gas, the wood chips aregenerally fed to the bin from its upper end and the steamed wood chipsare removed from the lower end of the bin. The steam is generally fed tothe lower part of the bin and it flows upwards against the direction ofmotion of the wood chip material. This kind of a solution for steamingwood chips is disclosed, for example, in the U.S. Pat. No. 4,867,845,where wood chips are fed to a bin with an downward-increasing diameterfrom the upper end of the bin and the steamed wood chips are removedfrom the lower end of the bin, by means of a rotating dischargerinstalled at the bottom of the bin. Steam is fed vertically to thecentre of the bin, via a steam pipe installed parallel to the verticalcentral axis of the bin. The end of the steam pipe extending close tothe bottom of the bin, from where the steam is discharged to the bin, isformed as an expanding taper. Steam flows upwards from the steam pipe,through the wood chip column packed in the bin.

The publication U.S. Pat. No. 6,199,299 also discloses a chip bin, wheresteam is fed to wood chips flowing downwards in the bin from the middleof the height of the bin, via steam nozzles installed in the shell ofthe bin.

US-publication 5,628,873 discloses a downwards-tapered steam bin, wheresteam is fed to the wood chip flow flowing downward in the chip bin fromabove the conical narrowing part. Steam feeding takes place both via theshell of the bin and the steam feeding pipes installed within the bin.

The problem with the above-mentioned bins is that the gas removal inthem is not even throughout the entire wood chip flow. The steam fed viathe bin shell cannot penetrate to the middle of the wood chip flow insuch a manner that all the wood chips would receive an identical steamprocessing. The steam fed to the middle of the bin, to its lower part,does not penetrate into the wood chips next to the walls of the bin orinto the chips on the wood chip surface of the bin efficiently enough toattain an even gas removal result. In addition, in the solutionspresented above, the warming of the wood chips is slow in the bin, inwhich case a part of the wood chips do not receive a proper gas removalprocessing.

BRIEF DESCRIPTION OF THE INVENTION

The purpose of the present invention is therefore to provide a methodfor removing gas from wood chips, which avoids the above-mentionedproblems and in which the gas removal from wood chips takes place insuch a manner that all the wood chips receive an identical steamprocessing. Thus, the pulp resulting from the cooking following the gasremoval is homogenous.

To attain this purpose, the method according to the invention isprimarily characterized in what will be presented in the characterizingpart of the independent claim 1.

The device according to the invention, in turn, is primarilycharacterized in what will be presented in the characterizing part ofthe independent claim 12.

The other, dependent claims present some preferred embodiments of theinvention.

The invention is based on the idea that gas removal from wood chips ispreformed in a vertical, elongated gas removal vessel, to which the woodchips are fed from the upper part of the vessel and the wood chips areremoved from its lower part. Steam is fed to the wood chips to the woodchip flow travelling downwards in the vessel from the middle and edgesof the flow, in cross-direction in relation to the wood chip flow. Thefeed rate of wood chips to the vessel and the discharge rate from it aremaintained on a level that the wood chip flow is moving as a so-calledplug flow, in which case the wood chips move evenly in relation to theentire cross-section area of the vessel. Therefore, steam is fed intothese wood chips in the plug flow. The level of the surface of woodchips in the vessel is controlled by means of the feed rate anddischarge rate of the wood chips. The surface of the wood chips ismaintained on such a level that the steam fed to the wood chips does notescape directly to the steam space in the upper part of the vessel. Thesteam feeding means are thus arranged to feed steam to a distance fromthe surface of the wood chips, inside the wood chip flow. The height ofthe gas removal vessel and the placement of the steam feeding means inrelation to the height of the vessel is arranged in such a manner thatthe delay time of wood chips in the vessel is long enough for the gasremoval to be as efficient as possible.

The majority of the steam required in the gas removal from wood chips isfed via the steam feeding pipe arranged in the middle of the wood chipflow and the rest of the steam is fed from around the wood chip flow,via steam feeding means arranged in the shell of the vessel. In orderfor the gas removal to be efficient enough, the temperature of the woodchips in the vessel is raised quickly to approx. 100° C. and maintainedthere substantially over the entire time the wood chips remain in thevessel. The amount of the steam being fed is controlled by means of thetemperature and pressure measurements of the steam gathering in theupper part of the vessel. The steam forming in the gas removal isremoved from the upper part of the vessel, from above the surface of thewood chips and the condensate is removed from the lower part of thevessel.

An advantage of the gas removal method according to the invention isthat as a result of both the design of the gas removal vessel and thetransverse steam flow, which does not prevent the flow of the woodchips, an even, downward plug flow of wood chips is created, resultingin each wood chip receiving an equally long and identical steamprocessing. Consequently, the gas removal within individual wood chipsis efficient and fast. The transverse steam flow does not prevent thedownward flow of the wood chips either. Further, the quick raising ofthe temperature of the wood chips to the processing temperature with atransverse steam flow ensures enough delay time for the wood chips inthe processing temperature, which improves gas removal significantly.Also, the dimensions of the gas removal vessel and the placement of thesteam feeding means in relation to the height of the vessel improve gasremoval. The gas removal device used in implementing the method, i.e.the gas removal vessel is simple to manufacture and easy to install inits place and it takes little room in the cramped device environments ofpulp and paper mills.

The invention can also be applied in renewing equipment in factories,which manufacture pulp. Thus, acquiring a completely new gas removaldevice can be avoided and economical savings can be created.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 shows a schematic side view of a device according to theinvention,

FIG. 2 shows a schematic side view of another device according to theinvention and

FIG. 3 shows a schematic side view of a steam feeding device used in adevice according to an embodiment of the invention.

In FIGS. 1, 2 and 3, the same numerals refer to corresponding parts andthey will not be explained separately later on, unless required by theillustration of the subject matter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a device 1 for removing gas from wood chips. The device isplaced in a pulp and/or paper factory before the digester where thecooking of pulp takes place. The device comprises is an elongated, byits diameter downwards tapered and advantageously conical vessel 2installed in the vertical direction. The vessel can be any vesselsuitable for the purpose, such as, for example, a bin 2. The wood chipsare fed to the bin 2 by means of a feeding device 3, from the upper endof the bin. The feeding device 3 is, for example, a rotary vane feederor a screw. The wood chips can be fed directly to the bin 2 by means ofthe feeding device or, such as in the advantageous embodiment shown inthe figure, the wood chips are fed with the feeding device to a chippacker 4 arranged between the roof 19 closing the upper part of the binand the feeding device. The chip packer 4 spreads the wood chips as aneven layer to the gas removal vessel 2 by means of the steam fed to thechip packer. The steam fed to the packer also increases the temperatureof the wood chips before they fall to the bin 2. In the winter, if thewood chips are frozen, the steam fed to the chip packer also melts thewood chips. The amount of steam fed to the chip packer 4 is controlledby the temperature of the wood chips. The operation of the chip packeris obvious to a man skilled in the art, which is why it is not describedmore in detail here.

The wood chips that have gone through the gas removal process areremoved from the bin 2 by the discharge means 5 that are connected tothe bottom of the bin. The discharge means comprises several pushingmeans 6, which move the wood chips to a discharge opening 7 that iscentralized with the midpoint of the bottom of the bin 2. The pushingmeans 6 are placed substantially evenly over the periphery of the entireshell of the bin 2 defining the bottom of the bin. The powertransmission of the discharge means is implemented, for example,hydraulically. The discharge opening is connected to a discharge meansarranged under it, i.e. to a screw 8, by means of which the gas-freewood chips are transferred forward.

The wood chips fed to the bin fill the bin 2 substantially evenly to thelevel of the surface 9 of the wood chips. The surface of the wood chipssettles within a distance from the roof 19 of the bin. A free space,i.e. a gas space 13 remains between the surface 9 of the wood chips andthe roof of the bin. The level of the surface 9 of the wood chips in thebin is controlled by controlling the feed rate of the wood chip feedingdevice 3 and/or the discharge rate of the wood chip discharge means 5and the screw 8 discharging the wood chips from the bin. The dischargerate is controlled according to the production of the digester. For thepurpose of controlling, one or more sensors 10 measuring the level ofthe surface of the wood chips are arranged in the bin 2. On the basis ofthe data provided by the surface level sensor 10, the controller 21controls the rate of the feeding device 3 and/or the discharge means 5and the screw 8 and at the same time the delay time of the wood chips inthe delay zone 25 between the steam feeding means 11 and 12 as well asthe discharge means 5. The surface level of the wood chips is to be thewood chips must be controlled onto such level that the wood chip surfaceextends all the time above the steam feeding openings of the steamfeeding means 11 and 12 feeding steam to the bin. Thus, the steam isprevented from directly escaping to the steam space 13 above the surfaceof the wood chips.

As can be understood from what is described above, the wood chips movedownward in the bin 2 by forming a downward moving wood chip flow, whichis described with the arrow A in the figure. The downward movement andthe effect of the steam fed to the wood chip flow causes the wood chipflow to become dense, thus forming a so-called plug flow. Thus, thedownward expanding design of the bin promotes the even movement of theflow in such a manner that no blockages or other obstacles blocking theeven flow form in it. The wood chip flow is guided by means of the flowcontroller 14 arranged in the lower part of the bin, which flowcontroller evens the chip flow in the bin. In addition, the flowcontroller 14 prevents the wood chips from directly discharging to thedischarge opening 7 and thus the blockage of the discharge opening 7.

Steam is fed to the wood chip flow in cross direction in relation to thewood chip flow with first steam feeding means 11 and second steamfeeding means 12. The first steam feeding means 11, i.e. the steamfeeding pipe 11 is arranged in the middle of the bin 2. The steamfeeding pipe 11 thus directs steam to the middle of the bin 2 in such amanner that at least a part of its length combines with the verticalmid-axis of the bin. The end of the steam feeding pipe 11 that extendsinside the bin is closed, preventing the steam from flowing into thechip flow directly, in the same direction with it. In the lower end ofthe steam feeding pipe 11 there is an around the steam feeding pipeextending steam distribution means 15, which can be a perforationextending around the feeding pipe, or some suitable means attached tothe feeding pipe, such as, for example, a screen, which distributessteam from the steam feeding pipe 11 evenly to the wood chipshorizontally on every side of the pipe. In the embodiment of FIG. 1, ascreen 15 for dividing steam is arranged in the steam feeding pipe. Thesteam feeding pipe 11 thus feeds steam to the middle of the bin in across-direction in relation to the wood chip flow, which steam is markedwith arrows in the figure. In the shell of the bin, substantially on thesame level with the screen 15, there are the second steam feeding means12 extending around the entire shell of the bin. The second steamfeeding means comprising a distribution chamber 12 and a steamdistribution means 20 direct steam to the bin in a cross-direction inrelation to the wood chip flow. The distribution means 12 is arranged onthe outer surface of the bin and it extends around the entire bin shell.The steam distribution means 20 is, for example, a screen and it isarranged on the inner surface of the bin shell, in connection with thedistribution chamber and also extends around the bin shell. Thecross-directional steam flow enables the quick raising of thetemperature of the wood chips to a temperature that is advantageous forgas removal. It has been experimentally established that gas removalfrom wood chips takes place in approx. 100° C. The wood chips arequickly heated by means of the steam directed to the bin to thistemperature and are maintained in this temperature by means ofcontinuous steam feeding. The majority of the steam required in gasremoval is fed from the steam feeding pipe 11. The rest of the steam isfed from the steam feeding means 12 and/or the steam feeding units 17arranged on the bottom of the discharge means 5. It is also possible tofeed steam to the bin only from the steam feeding pipe 11, if the steamamount coming through it is enough to maintain a high enough temperaturerequired in the gas removal. Controlling the gas removal temperature isdone by controlling the amount of steam fed to the bin. The amount ofsteam is controlled by measuring the temperature prevailing in the delayzone 25 below the second steam feeding means 12 by means of a controller23, which sends a control message to the steam feeding means (not shownin the figure). The control message is in the figure referred to withthe letter B and dashed lines. It has been experimentally established,that the delay time of an individual wood chip piece in the delay zone25 should be approximately 10 to 30 minutes, advantageouslyapproximately 15 to 25 minutes in order for the gas removal from it tobe as complete as possible. The steam feeding means 11 and 12 are placedin relation to the height of the bin 2 in such a manner that below thesecond steam feeding means 12, between the steam feeding means 11 and 12and the wood chip discharge means is formed a so-called delay zone 25,where an adequate delay time, which is required for gas removal, isguaranteed for the downward flowing wood chips. The delay zone isapproximately ⅔ of the height of the vessel 2.

The gases that the steam has displaced from the wood chips and thespaces between them, gathers in the gas space 13 between the surface 9of the wood chips and the roof 19 of the bin, from where it isadvantageously directed out via a unit 16. It is also possible to removegases from the bin 2 by sucking them via the steam feeding pipe 11 orthe distribution chamber 12. Thus, the steam flow can be effectivelydirected in a cross-direction through the wood chip flow. In addition,this way the channelling of the steam in the wood chip flow can beprevented. The steam gases can be sucked from only one of the steamfeeding means at a time, because feeding enough steam to the wood chipflow and maintaining a high enough temperature in it must be guaranteedthroughout the entire gas removal process. In other words, if the steamgases are sucked via the distribution chamber 12, enough steam must befed from the steam feeding pipe 11. Further, if the sucking of steamgases takes place via the steam feeding pipe 11, enough steam must befed via the distribution chamber 12 and the steam feeding units 17.

By means of the temperature sensor 22 arranged in the gas space 13 abovethe wood chip surface 9, the temperature of the gas space 13 above thewood chip surface is monitored. If the temperature rises too high, it isa sign of a malfunction of the bin, in which case an alarm is given. Thetemperature prevailing in the gas space 13 above the wood chip surfaceis lower than the processing temperature of the wood chips. This isbecause a part of the steam fed to the wood chips flows upstream in theupper part of the wood chip column towards the surface 9 of the woodchips and displaces the colder gas in the wood chips, which flowsupwards to the gas space 13.

The condensate formed in connection with the wood chip gas removal flowsto the bottom of the discharge means 5 connected to the lower part ofthe bin and are removed from there via the condensate removal units 18.The condensate can also be removed from the screw 8 (not shown in thefigure).

FIG. 2 shows another embodiment of the gas removal device 1 according tothe invention. This embodiment differs from the above-describedembodiment of FIG. 1 for the part of the wood chip discharge means. Gasremoval from the wood chips takes place in a corresponding manner as isdescribed in connection with FIG. 1. In FIG. 2 the downward flowing woodchip flow formed by wood chips is removed from the bin 2, such as in theembodiment of FIG. 1, by means of the wood chip discharge means 5 inconnection with the bottom of the bin 2. There are several hydraulicallymoving pushing means 6 in the discharge means, which move the wood chipsfrom one side of the bin to the discharge opening 7 placed on the otheredge of the bin 2. The screw 8 moves the gas-free wood chips forward. Inthis embodiment no separate flow controller is required for directingthe wood chip flow to the discharge means 5.

The flow of the wood chips in the bin 2 can, if desired, be directed bymeans of the design of the steam feeding means 11. FIG. 3 shows an inthe bin 2 arranged steam feeding pipe 11 having a closed, downwardconvergent tapered flow guide 24 attached to the lower surface of ascreen provided in the steam feeding pipe. The effect of the flow guideis based on that it prevents the sudden expansion of the freecross-sectional area of the bin after the screen 15 and thus evens thewood chip flow downwards.

The invention is not intended to be limited to the embodiments presentedas examples above, but the invention is intended to be applied widelywithin the scope of the inventive idea as defined in the appendedclaims. The gas removal vessel can thus also be of the same diameter inrelation to its entire height, a vessel that is round in its crosssection. Further, as the steam used in gas removal can be used eitherfresh steam or expansion steam received from another process of the pulpor paper mill. Also, the steam gas received from the gas removal devicecan be compressed and recycled back as heating steam to the device.

1-19. (canceled)
 20. A method for removing gas from wood chips, in whichmethod the downwards flowing wood chips are heated with a steam flowthat is cross directional in relation to the wood chip flow and isdirected to the wood chip flow both from the middle of the wood chipflow and from the edges of the wood chip flow, wherein the wood chipsare heated to a gas removal temperature, after which heating the woodchips are maintained in the gas removal temperature for a certain delaytime.
 21. The method according to claim 1, wherein the wood chips areheated in a vertically installed, elongated vessel, where steam isdirected to the wood chip flow by means of first steam feeding meansarranged in the middle of the wood chip flow and second steam feedingmeans arranged in the shell of the vessel.
 22. The method according toclaim 21, wherein the wood chips are fed to the vessel from the upperend of the vessel and removed from the bottom of the vessel, by means ofwood chip discharge means in connection with the bottom of the vessel.23. The method according to claim 22, wherein the level of the surfaceof the wood chips fed to the vessel is controlled by controlling thefeed rate of the wood chips and the discharge rate of the wood chips.24. The method according to claim 20, wherein the delay time of the woodchips after the heating is approximately of approximately 10 to 30minutes, advantageously 15 to 25 minutes.
 25. The method according toclaim 20, wherein the delay time of the wood chips fed to the vessel inthe delay zone is controlled by controlling the feed rate of the woodchips and the discharge rate of the wood chips.
 26. The method accordingto claim 21, wherein the steam feeding means are arranged in relation tothe height of the vessel in such a manner that a delay zone is formedbelow the steam feeding means in the flow direction of the wood chips.27. The method according to claim 20, wherein the temperature of thewood chips in the vessel is controlled by controlling the amount ofsteam directed from the steam feeding means to the vessel.
 28. Themethod according to claim 20, wherein gas is removed from the wood chipsbefore the wood chips are directed to cooking.
 29. A device for removinggas from wood chips, which device comprises a vessel, means for feedingwood chips to the vessel from its upper end, wherein the wood chips arearranged to flow downward in the vessel, and steam feeding means forfeeding steam in a cross-direction in relation to the wood chip flow inorder to heat the wood chips and to remove gas from it, wherein thesteam feeding means are arranged to direct steam to the wood chip flowfrom the middle of it and from its edges, wherein the wood chips arearranged to be heated to a gas removal temperature, after which heatingthe wood chips are arranged to be maintained in the gas removaltemperature for a certain delay time.
 30. The device according to claim29, wherein the steam feeding means contain first steam feeding means,which are arranged in the middle of the wood chip flow flowing in thevessel, and second steam feeding means, which are arranged in the shellof the vessel.
 31. The device according to claim 29, wherein the steamfeeding means comprise steam distribution means for directing steam in across-direction in relation to the wood chip flow.
 32. The deviceaccording to claim 29, wherein the device contains wood chip dischargemeans for discharging wood chips from the vessel, from the lower end ofthe vessel.
 33. The device according to claim 29, wherein the steamfeeding means are arranged in relation to the height of the vessel insuch a manner that a delay zone is formed between the steam feedingmeans and the wood chip discharge means.
 34. The device according toclaim 33, wherein the delay time of the wood chips in the delay zone isof approximately 10 to 30 minutes, advantageously 15 to 25 minutes. 35.The device according to claim 29, wherein the vessel is a verticallyinstalled elongated vessel.
 36. The device according to claim 29,wherein the device comprises a surface level sensor, which is arrangedto measure the level of the surface of the wood chips in the vessel, anda controller, which is arranged to control the level of the surface ofthe wood chips by controlling the discharge rate of the wood chipdischarge means and the feed rate of the wood chip feeding means on thebasis of the data provided by the surface level sensor.
 37. The deviceaccording to claim 29, wherein the controller is arranged to control thedelay time of the wood chips in the delay zone by controlling thedischarge rate of the wood chip discharge means and the feed rate of thewood chip feeding means on the basis of the data provided by the surfacelevel sensor.
 38. The device according to claim 29, wherein the devicecomprises a controller, which is arranged to measure the temperature ofthe wood chips in the vessel and to control the amount of steam fed tothe steam feeding means.